The Grammy-winning musician swaps his signature fedora for a gynecologist's lab coat to play Dr. Pomatter through Jan. 12.

Jason Mraz once acted in a high school play. It was a murder mystery, and his role was a janitor who mentioned a pivotal clue. During one performance, that janitor never appeared.

“I was goofing around backstage and missed the scene entirely, and the other actor had to invent a monologue about what he and I would’ve talked about,” he recalls. “I was horrified, and my teacher was livid. I learned that there are no small roles, and I didn’t act much after that.”

The Grammy-winning singer-songwriter is taking another chance with acting by making his Broadway debut in Waitress, Sara Bareilles’ hit musical about a pregnant diner server who hopes her extraordinary pies will lead to a better life. Playing her gynecologist and lover Dr. Pomatter, Mraz makes sure he never checks his phone between scenes: “That can take you out of the show so easily, and I vowed never to bring any of that high school shit into this.”

Mraz, 40, went Off Script with The Hollywood Reporter to talk conquering awkward humor, braving the New York City winter and finally recording new music.

You first sang Waitress tunes for Bareilles’ concept album of the show. Any apprehensions about fully taking on the role?

I didn’t know much about the character [when recording those songs]. She just gave me a summary of who he was, and hopefully my voice sounds mature enough that it’s believable to be a doctor! So I watched the film and saw how soft-spoken he is, and got on board. Plus, the thrill of doing two duets with Sara, that’s a no-brainer.

I’m so grateful to be associated with the project, watching it grow and debut on Broadway. When Sara called me to step into the role, I thought, this is gonna be a stretch — I’ve never acted in my adult life except for some sketch comedy with friends — but it didn’t take me more than 30 seconds to know that I should do it. As soon as I arrived in rehearsals, I knew I was in a good hands. It’s been fun.

What do you like most about Dr. Pomatter?

I’ll tell you what I don’t like — that he allows himself to break his loyalty and fidelity with his wife. But I do like that he allows himself to be disarmed and taken on this reality check of an adventure that suddenly seems to shake him up. It’s done that for me, in a way; it’s gotten me out of my routine for the past 15 years and woken me up in both my career and my personal life.

You’re in the musical through Jan. 14. As a longtime California resident, how are you handling the NYC weather?

I’m excited. I think the cold keeps you young. I was late to this interview because I was buying more layers; I was trying on a sweater. I once lived in New York — I was 18 years old, and it was where the seed was planted to be an entertainer. It was thrilling, but I was working at Barnes & Noble to make ends meet and I thought, "I’ll come back when New York wants me and gives me a job." Twenty-two years later, here I am.

What have you given up for the role?

Dr. Pomatter is off sugar, so I thought I would be, too, except for the three moments in the show when he’s eating Jenna’s pie. So that’s my reward every night onstage. Luckily it’s eight shows a week.

What’s the toughest part of the show for you?

He’s goofy, clumsy and nervous. There are a couple of jokes where he’s at the bus stop, very awkwardly having nervous banter with Jenna. To pull that off and try to get a joke out of it is the challenge. Some audiences love it and some are very quiet. Luckily the scene is awkward anyway. It’s great to ride that energy — on opening night, I was so nervous, but the doctor is nervous, too, so nobody knew.

What’s your impression of the stage door?

It’s a lot more organized — there’s an actual door! On tour, you don’t know if [fan meet-and-greets are] gonna be in a hotel lobby or on the bus. That’s more random and less secure.

Would you ever write a musical?

She made it look easy! I don’t know. Honestly, a couple years ago, I was ready to make my last record and be a farmer, and that still looks pretty good. But I also do still love music. Who knows.

In fact, you have a new song in the Warner Bros. comedy Father Figures, starring Owen Wilson and Ed Helms, and out Dec. 22.

Yeah! I love those guys. It’s a bromance about brotherhood, and what love can do. That’s my reoccurring thing as a songwriter: what love can do. It’s been three years since I realized an album, so I hope fans will be excited to hear that there’s a new song of mine out there in the universe after all.